You would probably take the greater and they would advance as per whether they would classify as minor psychics (1d6 per level), major psychics (1d6 +1) or master (check the books on that one).
I dont know much about ATB characters but I figure that maybe I would throw in the most reasonable balanced answer.
ATB animals in Rifts - psionics & ISP
Moderators: Immortals, Supreme Beings, Old Ones